A fuel cell is an effective device that can convert chemical energy to electric energy through electro-catalytic reactions. The proton exchange membrane fuel cell (PEMFC) operates at a relatively low temperature with gas phase hydrogen as fuel and oxygen (air) as oxidant. Due to its high energy conversion efficiency, low noise and low chemical emissions, the PEMFC demonstrates high potential in automobile and distributed power generation.
At the core of a PEMFC is the membrane electrode assembly (MEA) which consists of an anode, a cathode and a polymer electrolyte layer in between. At the surface of the anode, a hydrogen molecule is oxidized to two protons through the electro-catalytic process:H2→2H++2e−  (1)
The protons thus produced are transported to the cathode through the proton conductive membrane. At the surface of the cathode, oxygen is electro-catalytically reduced and subsequently reacts with protons from the equation (1) to form water, as follows:O2+4e−+4H+→2H2O  (2)
Reaction (2) is also known as the oxygen reduction reaction (ORR). Reactions (1) and (2) occur on the surface of the electrode catalysts. At present, the most effective catalyst for these reactions are made of platinum supported on an amorphous carbon. A typical Pt weight loading on the catalyst support ranges from 15% to 40%. Since platinum is a precious metal with limited supply, its usage adds a significant cost to a PEMFC system. Furthermore, the current method of preparing a MEA is very ineffective in utilizing platinum. An ink containing Pt/carbon catalyst mixed with a polymer solution (ionomer) is cast on the surface of the membrane, followed by hot pressing. Such a method often buries Pt/carbon catalyst particles underneath the ionomer matrix rendering them inaccessible to hydrogen or oxygen and unavailable to participate the aforementioned reactions. Fully utilizing the active catalyst is very important in reducing cost, especially for the cathode application since ORR is a more sluggish reaction than hydrogen oxidation, thus often requiring more catalyst. For example, the amount of platinum used at the PEMFC cathode typically is around 0.4 mg/cm2 whereas that used at anode is about 0.14 mg/cm2.
There are numerous methods existing in preparing conventional noble metal based electrode catalysts for fuel cell application. A brief summary was provided by Wilson and Gottesfeld as disclosed in the Journal Of Applied Electrochemistry 22, Wilson and Gottesfeld, the disclosure of which is incorporated herein. The inventive method is superior than the prior art because a) ACNTs according to the invention has good electro-catalytic activity yet does not have to contain noble metal; b) ACNTs according to the invention have unique shapes, orientation and spatial patterns for alignment and bundling that are not possible by the conventional electrode catalyst materials.
Dodelet and coworkers as disclosed in Electrochim Acta 48, M. Lefevre et al. and Electrochim Acta 43, G. Faubert et al., the disclosures of which are incorporated herein by reference, have published a series of studies on preparing noble metal free electro-catalyst for ORR using the macromolecules containing a functional group with transition metal coordinated by four pyrrolyl nitrogens, MN4.
The catalysts were fabricated by mixing or impregnating a macromolecule with MN4 group such as Fe porphyrin or FePc over a carbon precursor or carbonaceous materials, followed by high temperature treatment in hydrogen, argon and ammonia gas. The powder materials after high temperature treatment were collected as electrode catalysts. The subject invention is fundamentally different from the prior art based on the following key differences; (a) the inventive catalysts are prepared through CVD process, i.e. the precursor is first vaporized then re-deposited to form over a substrate. Therefore, the inventive method provides better mixing of organometallics and hydrocarbons and wider metal-to-carbon ratios than that of the prior art, (b) ACNTs produced according to the subject invention has a graphitic structure with ORR catalytic active sites embedded longitudinally in the surface of ACNTs, providing better stability in acidic and oxidative environments than the catalyst from the prior art where carbon is in the amorphous form and is unstable under these conditions, (c) ACNTs produced according to the subject invention has unique tubular shape with identical spatial alignments. The amorphous powder of the prior art do not have such properties.
H. Tang et al. discussed a method of dispersion platinum over aligned carbon nanotubes to generate electrocatalytic properties, H. Tang et al., Carbon 42 (2004) 191, the disclosure of which is incorporated herein. The instant invention is superior to this method since the inventive catalyst does not have to contain costly noble metals such as platinum.